4

The term usec appears in the delay field of the show interface command on a router:

Router# show interfaces fastethernet 0
 Fast Ethernet0 is up, line protocol is up 
   Hardware is DEC21140, address is 0000.0c0c.1111 (bia 0002.eaa3.5a60)
   Internet address is 10.0.0.1 255.0.0.0
   MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255
                                   ^^^^^^^^^^^^

According to Wikipedia, a usec is a microsecond:

A microsecond is an SI unit of time equal to one millionth (0.000001 or 10−6 or 1/1,000,000) of a second. Its symbol is μs.

There are values associated with each interface, based upon the interface's speed:

  • An ethernet (10 Mbps) interface has a delay of 1000 usec.
  • A fast ethernet (100 Mbps) interface has a delay of 100 usec.
  • A gigabit ethernet (1000 Mbps) interface has a delay of 10 usec.

In effect, the lower the delay usec, the higher the interface speed. But what is delay really measure of? What goes into the calculation to determine that a 100 Mbps link has a delay of 100 usec (and so on)?

It can't be the time it takes for a "bit" to cross the wire, because voltage applied to any interface speed traverses at the speed of light. It can't be a measurement of how many bits can be sent over a given time frame (because that is the Bandwidth measurement).

So what is usec really a measure of?

  • 2
    μ (the Greek letter “mu,” pronounced myoo) is the SI abbreviation for “micro,” just like m for milli and k for kilo. Most keyboards outside Greece don't have a μ key (and it's not in ASCII), so it's common to type it as “u.” – Jacktose Apr 9 '16 at 12:17
1

That is a measurement of the time between bits as they are placed on the "wire." For instance, a 1 Gbps interface will send one frame every 10 microseconds, but a 100 Mbps interface does that 10 times slower. It is basically the serialization delay imposed by the interface.

This number is used by EIGRP in its metric calculations, as are all the numbers on the line:

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255

Interface Commands

Edit:

I found a Cisco document that explains more:

Setting Interface Delay

Higher-level protocols might use delay information to make operating decisions. For example, IGRP can use delay information to differentiate between a satellite link and a land link. To set a delay value for an interface, use the following command in interface configuration mode:

...

Setting the delay value sets an informational parameter only; you cannot adjust the actual delay of an interface using this configuration command.

This CCNA "Core Knowledge" document explains:

  • Setting bandwidth or delay on an interface does not change any physical properties of the interface at all; you are just changing the values that the interface reports for EIGRP metric purposes

and

The DELAY command is a powerful command for manipulating EIGRP paths. Since the BANDWIDTH command can end up impacting a lot of other configurations (like QoS), we can use the DELAY command to manipulate EIGRP metrics (and therefore, paths) without having to touch the BANDWIDTH command.

  • That sound awfully a lot like the frequency... but that doesn't seem right, because 10BASE-T has a frequency of 10Mhz, or 10,000,000 'instances' of voltage being applied per second to communicate the transfer of a bit. 100BASE-TX has 100 Mhz, and 1000BASE-T uses 125 Mhz. Doesn't seem to line up with the usec values. – Eddie Mar 19 '16 at 21:08
  • @Eddie, it is a somewhat arbitrary number. Cisco assumes an arbitrary frame size as a reference for this. It's mostly for comparison, much like OSPF cost, as a component of the EIGRP metric. – Ron Maupin Mar 19 '16 at 21:20
  • It has to mean something. It wouldn't be in "show interface" if not. I'm sure it means something outside of EIGRP. Going to leave this open for a few to see what else comes in. – Eddie Mar 20 '16 at 1:25
  • @Eddie, if it actually meant something outside the scope of EIGRP, you wouldn't be able to change it to any value as you can with the interface delay command. You could change the value from 10 on a 1 Gbps interface to 5 or to 5000. – Ron Maupin Mar 20 '16 at 1:29
  • @Eddie, I found a link with a table for the delay on common interfaces. This link shows that using the delay command changes what you see from the show interfaces command. – Ron Maupin Mar 20 '16 at 1:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.